ATTRIBUTES OF THE NITROGEN UPTAKE SYSTEMS OF MAIZE (ZEA-MAYS L) - MAXIMAL SUPPRESSION BY EXPOSURE TO BOTH NITRATE AND AMMONIUM

Higher plants are commonly exposed to rapid changes in the amounts and proportions of ammonium and nitrate in their root environment. Some o f the potential consequences of such changes have been determined by e xamining the suppressive effects of growing maize (Zea mays L.) seedli ngs with a high concentration (3 mM) of nitrogen on the subsequent upt ake of nitrogen from a more dilute (0.2 mM) solution. To document the interactive role of nitrate and ammonium supply in suppression, ratios of these ions in the growth solution were maintained at 3:0, 2:1, 1:2 and 0:3. Rates of nitrate and ammonium uptake were then measured duri ng initial (0-1 h) exposure to 0.2 mM KNO3 or NH4NO3 and hourly during a subsequent 7 h period of adaptation to the dilute solutions. Maxima l suppression of nitrate uptake occurred only when both nitrate and am monium were present (2:1 and 1:2) during prior growth. Nitrate uptake rates increased two- to three-fold as the seedlings adapted to the dil ute KNO, solution, but the presence of ammonium with nitrate in the pr ior growth solution restricted the rate and extent of recovery from su ppression. Recovery was further restricted when ambient ammonium was p resent during the adaptation period. Neither the magnitude of suppress ion nor the rate and extent of recovery was readily explained by (a) t he proportion of nitrate and ammonium present during prior growth, (b) carbohydrate concentrations in root and shoot tissues, (c) concentrat ions of nitrate and ammonium in the root tissue, or (d) indirect effec ts of ammonium on potassium uptake. Total nitrate reduction by the ent ire plant in the 8 h adaptation period decreased as the proportion of ammonium in the prior growth solution increased. Ambient ammonium rest ricted nitrate reduction only in seedlings previously grown entirely w ith ammonium. Thus the effect of ambient ammonium on nitrate reduction by the whole plant differed substantially from that on nitrate uptake . Since nearly all the ammonium taken up by the roots was assimilated, as well as that generated by nitrate reduction, it appears that carbo hydrate supply did not limit the observed uptake of ammonium and nitra te. The accumulation in roots of both nitrate and specific metabolites of ammonium serving as negative effecters could account for most of t he observations.